Tolvaptan represented by the following formula (2) is a known compound, and is disclosed, for example, in U.S. Pat. No. 5,258,510 (Example 1199).

Tolvaptan is known to be useful as a vasopressin antagonist having aquaretic efficacy (Circulation, 107, pp. 2690-2696 (2003)). However, because of its low water solubility, tolvaptan has problems in that it is poorly absorbed from the gastrointestinal tract, and its dosage form and administration route are limited. From the viewpoint of medical treatment, the development of a new drug that can maintain for a long period of time the blood level of tolvaptan enabling to provide the desired pharmaceutical effects has been desired.